A microchannel plate is a key component of an image intensifier tube. Image intensifier tubes are employed for the purpose of amplifying a low intensity or non-visible radiational image of an object into a readily viewable image. Many industrial and military applications exist for such devices including enhancing the night vision of aviators, rendering night vision to persons who suffer from retinitis pigmentosa, more commonly known as night blindness and photographing astronomical bodies.
The general construction of a currently employed image intensifier tube is exemplified in FIG. 1 which illustrates a Generation III (GEN III) image intensifier tube 10. Examples of GEN III image intensifier tubes can be found in U.S. Pat. No. 5,029,963 to Naselli, et al., entitled REPLACEMENT DEVICE FOR A DRIVER'S VIEWER and U.S. Pat. No. 5,084,780 to Phillips, entitled TELESCOPIC SIGHT FOR DAYLIGHT VIEWING both of which are manufactured by ITT Corporation, the assignee herein.
The GEN III image intensifier tube 10 shown in FIG. 1 comprises an evacuated envelope or vacuum housing 22 having a photocathode 12 disposed at one end of the housing 22 and a phosphor screen 30 disposed at the other end of the housing 22. A microchannel plate (MCP) 24 is positioned within the vacuum housing 22 between the photocathode 12 and the phosphor screen 30.
The photocathode is comprised of a glass faceplate 14 coated on one side with an antiflection layer 16; a gallium aluminum arsenide (GaAlAs) window layer 17; a gallium arsenide (GaAs) active layer 18; and a negative electron affinity (NEA) coating 20.
The MCP 24 is located within the vacuum housing 22 and is separated from the photocathode 12 by gap 34. An MCP is an electron multiplier formed by an array of microscopic channel electron multipliers. The MCP 24 is generally made from a thin wafer of glass having an array of microscopic channels extending between input and output surfaces 26 and 28 respectively. The wall of each channel is formed of a secondary emitting material. The phosphor screen 30 is located on a fiber optic element 31 and is separated from the output surface 28 of the MCP 24 by gap 36. The phosphor screen 30 generally includes aluminum overcoat 32 to stop light reflecting from the phosphor screen 30 from re-entering the device through the NEA coating 20.
In operation, infrared energy coming from an external object impinges upon the photocathode 12 and is absorbed in the GaAs active layer 18, resulting in the generation of electron/hole pairs. The electrons generated by the photocathode 12 are subsequently emitted into gap 34 of the vacuum housing 22 from the NEA coating 20 on the GaAs active layer 18. The electrons emitted by the photocathode 12 are accelerated toward the input surface 26 of the MCP 24 by applying a potential applied across the input surface 26 of the MCP 24 and the photocathode 12 of approximately 800 volts.
When an electron enters one of the channels of the MCP 24 at the input surface 26, a cascade of secondary electrons is produced from the channel wall by secondary emission. The cascade of secondary electrons are emitted from the channel at the output surface 28 of the MCP 24 and are accelerated across gap 36 toward the phosphor screen 30 to produce an intensified image. Each microscopic channel functions as a secondary emission electron multiplier having an electron gain of approximately several hundred. The electron gain is primarily controlled by applying a potential difference across the input and output surfaces of the MCP 24 of about 900 volts.
Electrons exiting the MCP 24 are accelerated across gap 36 toward the phosphor screen 30 by the potential difference applied between the output surface 28 of the MCP 24 and the phosphor screen 30. This potential difference is approximately 6000 volts. As the exiting electrons impinge upon the phosphor screen 30, many photons are produced per electron. The photons create an intensified output image on the output surface of the optical inverter or fiber optics element 31.
The image reproducing effectiveness of prior art MCPs depends in part on the ability of the cascading electrons coming from each channel of the MCP 24, to reach the phosphor screen 30 before any significant spatial dispersion occurs. If the cascading electrons spatially disperse before reaching the phosphor screen 30, the resolution of the intensified image will become degraded.
Spatial dispersion is prevented in current image intensifier tubes by locating the phosphor screen in very close proximity to the output surface of the MCP, and applying a high electric field to the gap 36. This approach has many drawbacks resulting from the employment of high electric fields and close MCP/phosphor screen spacings, including field emission, breakdown, and in extreme cases, pull-off of the aluminum overcoat on the phosphor screen. Ultimately, the result of these drawbacks is the loss of the image intensifier tube.
Therefore, there exists a need in the art of image intensifier devices for an image intensifier that avoids the problems of prior art devices without the usual loss of resolution.
It is therefore, a primary objective of the present invention to provide an image intensifier tube that utilizes a focussed-output MCP (FMCP) to provide an image upon a phosphor screen within a vacuum housing while providing state of the art performance.